Accompanied by development of sciences and technologies, people are in increasing pursuit of more extensive field of view and higher resolution. This is especially important in the micro-imaging field. A good structure of objective lens has tremendous influence on micro-imaging effect.
A common micro-objective lens is inadequate to ensure both field of view and resolution; whereas, improvement of imaging quality is extremely important to the whole microscopic system. The resolution of microscopic system is mainly determined by the diffraction limit of objective lens. Therefore, design of objective lens is a critical part of the whole microscopic system.
Numerical aperture of the micro-objective lens is an important optical parameter of a microscope. As indicated by Rayleigh criterion, resolution of microscopic system can be represented by σ=0.61λ/NA; wherein, λ refers to optical wavelength; NA refers to numerical aperture; light of short wavelength can improve resolution of the microscopic system. To improve resolution of the microscopic system under the light of the same wavelength, it is necessary to increase numerical aperture (NA). Numerical aperture of microscope can be represented by NA=n sin α. On this account, it is applicable to use oil immersion objective lens to increase numerical aperture value. However, this may make it more difficult for application. Furthermore, numerical aperture of objective lens is determined by the correction of aberration to the optical system to a great extent. We often use fluorite lens of low refraction and low chromatic dispersion to improve color difference and imaging quality of the lens.
Presently, optical microscope plays an important role in numerous fields, such as biomedicine. In the 20th century, resolution of optical microscope was up to nano scale. Nowadays, there are increasing demands for observation within extensive field of view, especially observation under fluorescence of extensive field of view of microscope. Furthermore, expansion of field of view of micro-objective lens and improvement of resolution serve as the critical link for design of micro-objective lens in the future.